Question

When is war justified? And what may justifiably be done during the conduct of a war?

Answer 1

A war wins only if it's waged for a legitimate cause, and that carries enough moral weight. The nation that wants to use military force must prove that there is a just cause to do so. The main reason is to correct a mistake. A battle waged to keep a wrong from occurring can also be called a just battle. In modern times battles to protect the innocent are generally considered just (which corresponds with the belief in some religious literature that defending an innocent is better than defending oneself).

Self-defence against an aggressor is the clearest example of a just cause. For instance, when an enemy has crossed your frontiers and invaded your country. So it doesn't take an actual invasion. The cases of self-defence below are not clearly just causes of war-whether or not they are depends on how serious a particular case is:
Prominent individual murder:-monarch or president
Attack on national honour: (e.g., flag burning, embassy attack)
Attack on State Religion
Financial assault:(Embargo or sanctions)
Assault on a friend or ally

Assisting a friendly nation, invaded.
Violations of human rights: Another common example is to remedy so serious a violation of human rights that force is the only sensible solution.
Punishing an act of aggression: Not everyone accepts this. Many people would say a punitive war will never be a fair war.